1. Field of the Invention
The present invention relates to a method of forming a film over the inner surface of a cylindrical member having a center bore and, more particularly, to a method of forming a hard carbon film for enhancing the abrasion resistance of the inner surface of a cylindrical member (part), such as a bushing, a piston cylinder and a linear bearing.
2. Description of the Related Art
A hard carbon film is black and has properties similar to those of diamond. That is, the hard carbon film has advantageous properties including a high mechanical hardness, a small friction coefficient when contacting other materials, a high electrical insulation property, a large thermal conductivity and a high corrosion resistance. Accordingly, there have been proposals for coating various devices, including various ornaments, medical instruments, magnetic heads, tools and such with a hard carbon film.
A hard carbon film is a hydrogenated amorphous carbon film having properties very similar to those of diamond and hence a hard carbon film is often called a diamond like carbon film (DLC film) or an i-carbon film.
It is possible to remarkably enhance the abrasion resistance of the inner surface which is in slidable contact with other members by forming the hard carbon film over the inner surface of a cylindrical member having a center bore like various bushes such as a guide bush which is mounted on an automatic lathe to support a rod-shaped work-piece sidably and rotatably, a piston cylinder and a linear bearing.
Accordingly, the following steps are taken for forming the hard carbon film over the inner surface of the cylindrical member set forth above using a conventional chemical vapor deposition process.
That is, as shown in FIG. 14, a cylindrical member 11 having a center bore 11a is placed in a vacuum vessel 13 provided with a gas inlet port 15 and a gas outlet port 17.
The vacuum vessel 13 is evacuated through the gas outlet port 17 by an evacuating means, not shown. Then, a gas which contains carbon is supplied into the vacuum vessel 13 through the gas inlet port 15 and the pressure in the vacuum vessel 13 is adjusted to a set pressure.
Thereafter, a positive DC voltage is applied to an anode 31 placed within the vacuum vessel 13 from an anode power source 27, and an AC voltage is applied to a filament 33 by a filament power supply 29. Further, a negative DC voltage is applied to the cylindrical member 11 by a DC power source 25. Thus, a plasma is produced in the vacuum vessel 13 to deposit a hard carbon film on the entire surface including the inner surface 11b of the cylindrical member 11.
The hard carbon film forming process shown in FIG. 14 uses the plasma produced by the DC voltage applied to the cylindrical member 11 and the plasma produced by the filament 33 energized by the AC voltage and the anode 31 energized by the DC voltage. Either the plasma produced around the cylindrical member 11 or the plasma produced around the filament 33 and the anode 31 contributes mainly to hard carbon film formation depending on the pressure in the vacuum vessel 13 during hard carbon film formation.
For example, when the pressure in the vacuum vessel 13 is 3.times.10.sup.-3 torr or above, the plasma produced around the cylindrical member 11 mainly contributes to the decomposition of the gas containing carbon to form the hard carbon film.
Although a hard carbon film can be formed uniformly over the outer surface of the cylindrical member 11, a hard carbon film formed over the inner surface 11b defining the center bore 11a is poor in adhesion, and inferior in qualities such as hardness. This is because a set voltage is applied to the whole of the cylindrical member 11, and the center bore 11a defines a space in which electrodes of the same potential are disposed opposite to each other, and thus the plasma prevailing in the center bore 11a causes an abnormal discharge called hollow discharge. A hard carbon film formed by hollow discharge is a polymer like film inferior in adhesion and apt to come off the inner surface 11b of the cylindrical member 11, and it has a relatively low hardness.
On the other hand, when the pressure in the vacuum vessel 13 is below 3.times.10.sup.-3 torr, the plasma produced in the neighborhood of the filament 33 and the anode 31 rather than the plasma produced around the cylindrical member 11 contributes mainly to hard carbon film formation.
Although a hard carbon film can be uniformly formed over the outer surface of the cylindrical member 11, the hard carbon film cannot be formed in a uniform thickness with respect to a direction along the axis of the cylindrical member 11 over the inner surface lib defining the center bore 11a.
Carbon ions produced by the plasma produced around the filament 33 and the anode 31 are attracted to the surface of the cylindrical member 11 by the negative DC potential of the cylindrical member 11 to deposit the hard carbon film over the surface of the cylindrical member 11.
The hard carbon film is formed by a chemical vapor deposition process when the pressure in the vacuum vessel 13 is above 3.times.10.sup.-3 torr, and the hard carbon film is formed by a physical vapor deposition process when the pressure in the vacuum vessel 13 is below 3.times.10.sup.-3 torr. Therefore, the thickness of the hard carbon film formed over the inner surface 11b of the cylindrical member 11 decreases from the open end of the center bore 11a downwards to the depth thereof, which occurs when forming a film by a physical vapor-phase epitaxial growth process, such as a vacuum deposition process, when the plasma produced around the filament 33 and the anode 31 contributes mainly to hard carbon film formation. Consequently, the hard carbon film cannot be formed with a uniform thickness over the entire inner surface 11b of the cylindrical member 11.
A thickness distribution in a hard carbon film formed over the inner surface of a cylindrical member will be explained with reference to FIG. 15. In the graph of FIG. 15, distance from the open end of the cylindrical member is measured on the horizontal axis and thickness of film formed over the inner surface of the cylindrical member is measured on the vertical axes. In the graph of FIG. 15, curve a indicates the variation of the thickness of the hard carbon film formed over the inner surface of the cylindrical member by the method described with reference to FIG. 14.
As is obvious from curve a in FIG. 15, the thickness of a hard carbon film formed by the conventional method (in case of formation of an intermediate layer) decreases sharply from 1.0 .mu.m at the open end of the bore to 0.2 .mu.m at a position 30 mm inward from the open end.
Since the hard carbon film has such a sharply changing thickness, even if the hard carbon film is formed over the inner surface of a cylindrical member, it cannot fully exercise its advantageous characteristics including high abrasion resistance and high corrosion resistance.